Unit 2 Alkanes
Key unit competence: Analyse the structure, properties, preparation and uses of alkanes to explain their behaviour, applications and importance.
Key ideas
- Alkanes (CnH2n+2) are saturated hydrocarbons; each carbon is sp3 hybridised (tetrahedral, 109.5°) with only σ bonds. Cycloalkanes have formula CnH2n.
- Preparation: hydrogenation of alkenes (Ni, 150 °C); decarboxylation of sodium salts with soda lime (CH3COONa + NaOH → CH4 + Na2CO3); Wurtz reaction (2CH3Cl + 2Na → C2H6 + 2NaCl); reduction of halogenoalkanes; cracking; Kolbe electrolysis of carboxylate salts.
- Physical properties: non-polar, insoluble in water, soluble in organic solvents; boiling point increases with chain length and decreases with branching (London forces).
- Combustion: complete gives CO2 and H2O; incomplete gives CO and C (soot). Large heats of combustion make alkanes good fuels.
- Free-radical substitution with halogens in UV light: initiation (Cl2 → 2Cl·), propagation (Cl· + CH4 → ·CH3 + HCl; ·CH3 + Cl2 → CH3Cl + Cl·), termination (two radicals combine, e.g. ·CH3 + ·CH3 → C2H6).
- Cracking (thermal or catalytic with zeolites) breaks long alkanes into shorter alkanes and alkenes, increasing petrol supply and producing ethene for plastics. Uses: fuels (LPG, petrol, diesel, kerosene), solvents, lubricants, wax, bitumen. Environmental issues: CO, soot, CO2 (global warming), unburnt hydrocarbons.
Common misconceptions
- "In free-radical substitution only CH3Cl forms." No: Further substitution gives CH2Cl2, CHCl3 and CCl4, and termination gives ethane.
- "Radicals are ions." No: Radicals are neutral species with an unpaired electron.
- "Cracking gives only alkanes." No: Cracking always produces at least one alkene (to balance hydrogen).
Worked examples
Example 1. Describe the mechanism of the reaction between methane and chlorine in UV light.
Solution
Initiation: UV light breaks Cl–Cl homolytically: Cl2 → 2Cl·.
Propagation: Cl· + CH4 → HCl + ·CH3; then ·CH3 + Cl2 → CH3Cl + Cl· (radical regenerated, chain continues).
Termination: radicals combine: Cl· + Cl· → Cl2, ·CH3 + Cl· → CH3Cl, ·CH3 + ·CH3 → C2H6.
Overall: free-radical substitution.
Example 2. Write equations for three methods of preparing ethane.
Solution
Hydrogenation of ethene: C2H4 + H2 → C2H6 (Ni, 150 °C).
Wurtz reaction: 2CH3Br + 2Na → C2H6 + 2NaBr (dry ether).
Decarboxylation of sodium propanoate: C2H5COONa + NaOH → C2H6 + Na2CO3 (heat with soda lime).
Practice questions
Define a free radical.
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An atom or group with an unpaired electron.
Common mistake: Saying 'a charged ion'.
Name the three stages of a free-radical chain reaction.
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Initiation, propagation, termination.
Common mistake: Including 'addition'.
Write the equation for the decarboxylation of sodium ethanoate.
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CH3COONa + NaOH → CH4 + Na2CO3.
Common mistake: Writing ethane as product.
Explain why alkanes are generally unreactive.
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Strong, non-polar C–C and C–H σ bonds.
Common mistake: Saying they have double bonds.
Give two products of cracking decane.
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e.g. octane and ethene (any alkane + alkene pair that balances).
Common mistake: Giving two alkanes only.
Why do boiling points decrease with branching?
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Smaller surface contact, weaker London forces.
Common mistake: Saying fewer atoms.
Write an equation for incomplete combustion of propane forming CO.
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2C3H8 + 7O2 → 6CO + 8H2O.
Common mistake: Unbalanced oxygen.
State one environmental problem of burning alkanes.
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CO2 (global warming), CO (toxic), soot, smog (any one).
Common mistake: Saying ozone formation is beneficial.
Write the initiation step for bromination of methane.
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Br2 → 2Br· (UV light).
Common mistake: Writing Br2 → Br+ + Br−.
Name the catalyst used in catalytic cracking.
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Zeolite (aluminosilicate).
Common mistake: Saying iron.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Alkanes are insoluble in water because they are: a. two chlorine radicals 2. Homolytic fission of Cl2 gives: b. highly exothermic 3. Alkanes are used as fuels because combustion is: c. initiation 4. The first step of free-radical chlorination is called: d. non-polar Show answer
1 → d (non-polar) · 2 → a (two chlorine radicals) · 3 → b (highly exothermic) · 4 → c (initiation)
- No attraction to polar water.
- Each atom gets one electron.
- Large energy release.
- Cl2 splits homolytically in UV.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
zeolite catalysts · hydrogenation with a nickel catalyst · 90° · 109.5°- Catalytic cracking uses .
- Ethane can be made from ethene by .
- The bond angle around carbon in alkanes is about .
Show answer
1: zeolite catalysts · 2: hydrogenation with a nickel catalyst · 3: 109.5°
- Aluminosilicates.
- C2H4 + H2 → C2H6.
- Tetrahedral.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Unburnt hydrocarbons in exhaust contribute to: a. photochemical smog 2. Ethane formed during methane chlorination comes from: b. CO2 3. Catalytic converters change CO into: c. Octane 4. Which alkane has the highest boiling point? d. a termination step Show answer
1 → a (photochemical smog) · 2 → d (a termination step) · 3 → b (CO2) · 4 → c (Octane)
- With NOx in sunlight.
- ·CH3 + ·CH3.
- 2CO + 2NO → 2CO2 + N2.
- Longest chain.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
CnH2n-2 · sodium in dry ether · a species with an unpaired electron · CnH2n- The Wurtz reaction joins halogenoalkanes using .
- The general formula of cycloalkanes is .
- A free radical is .
Show answer
1: sodium in dry ether · 2: CnH2n · 3: a species with an unpaired electron
- 2RX + 2Na → R-R + 2NaX.
- Ring uses two bonds.
- Very reactive.